Poisson distribution for large mean

Hi, I am using poisson distribution (boost/random/poisson_distribution.hpp), and have encountered a problem when using a large mean. Let's say I'm setting up my poisson distribution with mean = MEAN, and repicking a sequence of 1000 random numbers. If I average out these 1000 numbers, I would expect to get MEAN (or very close to it). This works OK for mean < 745. Afterwards, no matter how large I set the mean, the same sequence of numbers is generated, with average of 745. Any help would be greatly appreciated!! Thank you, Ksenia